1. Field of the Invention
The present invention relates to a fuel evaporative emission control system to be used in an internal combustion engine.
2. Prior Art
A conventional fuel evaporative emission control system is illustrated in FIGS. 6 and 7.
In FIG. 6, the system comprises a fuel tank 100, a canister 200, a fuel evaporative emission control valve 300 which is provided in the pipe for coupling the fuel tank 100 to the canister 200. The fuel evaporative emission control valve 300 prevents the fuel evaporative emission in the fuel tank 100 from discharging to the atmosphere.
That is, the fuel evaporative emission control valve 300 is opened or closed when it detects the intensity of the pressure in the fuel tank 100. When the fuel evaporative emission control valve 300 is opened, the fuel evaporative emission is supplied from the fuel tank 100 to the canister 200 through the fuel evaporative emission control valve 300.
FIG. 7 shows an arrangement of a typical fuel evaporative emission control valve which comprises a cup-shaped casing 22 having a canister communication port 21 and a collar 24 having a fuel tank communication port 23 in which the latter is retained by the former to form an integral casing. A valve 25 which permits the communication from the fuel tank side to the canister side and is brought into contact with a valve seat surface by a spring 26. A shell-type check valve 27 is disposed at the portion adjacent to the center of the valve 25 and permits the communication from the canister side to the fuel tank side when it is opened.
An operation of this fuel evaporative emission control valve will be described hereinafter. When the pressure inside the fuel tank 100 rises at a reference value by the generation of the fuel evaporative emission in the fuel tank 100, the pressure of the fuel evaporative emission exceeds the resiliency of the spring 26 to thereby open the valve 25. As a result, the fuel evaporative emission in the fuel tank 100 is supplied to the canister 200 so that the pressure in the fuel tank 100 is lowered.
When the pressure in the fuel tank 100 is lowered due to the fuel consumption or due to a cold weather, or the like, the shell-type check valve 27 is opened so that the atmospheric air is introduced from the canister 200 to the fuel tank 100, which prevents the fuel tank 100 from deforming.
However, the conventional fuel evaporative emission control system has the following drawback. That is, since the valve 25 is not opened until the pressure in the fuel tank 100 reaches the reference value, the fuel evaporative emission which remains in the fuel tank 100 is discharged into the atmosphere when a filler cap is opened at the time of fuel supply in case the pressure in the fuel tank 100 does not reach the reference value.
To solve the aforementioned drawback, if resiliency of the spring 26 is weakened, the valve 25 is opened by fluid pressure of the fuel, whereby the fluid fuel tends to introduce into the canister 200.